Current resin transfer molding (RTM) processes are used to produce Fiber Reinforced Polymer (FRP) composite materials by infusing resins into different types of fiber reinforcement (Beckwith and Hyland, “Resin transfer molding: A decade of technology advances”, SAMPE Journal, Vol. 34, No. 6, November-December, pp. 7-19 (1998)). RTM methods are characterized by resin infusion of fiber reinforcement, fabrics or preforms within a closed mold or tool. RTM methods have the advantage of minimizing void content, producing high fiber volume content and controlling volatile organic compound (VOC) emissions. Resin infusion is attained by a pressure gradient that can be developed in three different ways: 1) by vacuum; 2) by external pressure and/or gravity; and 3) by a combination of vacuum and pressure.
Among the family of RTM processes, the subset known as Vacuum Assisted Resin Transfer Molding (VARTM) has shown great potential for fabricating FRP composite parts (Beckwith and Hyland 1998). However, the VARTM process requires two stiff molds to produce composite laminates. Construction of a Modular fiber reinforced polymer composite structural panel system using the VARTM process is presented in U.S. Pat. No. 6,309,732.
The SCRIMP™ process constitutes an improvement over VARTM for fabrication of large composite parts. In the SCRIMP™ process only a one sided tool is required and a vacuum is applied to infuse the fiber reinforcement inside a vacuum bag. The SCRIMP™ technology relies upon the controlled flow of resin through an in-plane distribution system. The resin distribution system allows dry fiber reinforcement layers to be infused with resin throughout the cross-sectional thickness. Two SCRIMP™ patents, U.S. Pat. No. 4,902,215 and No. 5,052,906, specifically address the use of a flow medium fed by a “pervious conduit” (a resin feed or channel) communicating with the flow medium (TPI Technology, Inc. (2001). An Overview of the SCRIMP™ Technology, Warren, R.I.). The SCRIMP™ technology also includes the use of core materials with resin flow features. This technology is described in U.S. Pat. Nos. 5,721,034, 5,904,972, 5,958,325 and 6,159,414.
The quality of composite parts prepared by art-recognized methods is dependant on several processing parameters (e.g., resin distribution media, vacuum setup, part geometry and thickness, resin/catalyst chemistry). Since, the maximum vacuum that can be applied in the SCRIMP™ process is approximately 1 Atmosphere (30 in of Hg or −15 psi), the ability of the technique to infuse FRP composite laminates and hybrid composite parts with different substrates is limited based on the distribution media. Furthermore, since the SCRIMP™ process requires dedicated labor to properly set up the distribution media and seal the vacuum bag avoiding gas leaks, it is relatively slow and expensive for mass production of composite parts.
One group (Larsen et al. AIAA-2002-0026) has employed a 2-stage process using a bag and, two-part mold to produce an FRP specifically for wind turbine blades. In the first stage of their process, a vacuum is applied similarly to the SCRIMP™ process to draw in and infuse an article or part with resin. The article or part is not consolidated to allow better resin infiltration into the fabric of the FRP. In a second stage, the two parts of the mold are closed and a low pressure of 55-10 kpa (8-15 PSI) is applied to the impregnated fabric to further distribute the resin along the length of the article. Higher clamping or consolidation pressures applied were said to reduce performance of the system, and were prohibitive given the design considerations of the mold used. The Larsen system does not use an initial application of pressure to infuse resin into a substrate or reinforcing layer. Moreover, the Larsen system is not disclosed to be applicable to systems that do not make use of a two-part mold or form.
In general, RTM processes can be modified so that the fiber reinforcement can be bonded to other substrate or core materials to develop hybrid composite products. In this case, the polymeric matrix serves both as a fiber binder and adhesive to a substrate. The substrate is not infused with the resin but rather the polymeric matrix bonds to the substrate surface. Examples of substrate or core materials are foams, cellular materials, ceramics, steel, wood products and Portland cement concrete. When these hybrid composite products are used in structural applications, failure typically develops at the substrate/FRP composite interface.
The present methods of fabricating resin infused composites are hampered by their inability to form graded interfaces between components of composite materials and also because vacuum systems create defects in the final material as localized low pressures cause microvoids due to volatilization of solvents in the resin. Moreover, currently used methods, e.g., RTM, require the use of expensive and complicated fabrication equipment. Thus, a simple, inexpensive method for forming a composite material with a graded interface and improved quality, would substantially improve the art of composite material formation.